Reduction of trace elements to the elemental form by microorganisms

ABSTRACT

A process for reducing the concentration of ionic species of heavy metals in an aqueous waste solution by conversion to a corresponding elemental metal comprises contacting the waste solution containing ionic species of one or more heavy metals with a culture of Pseudomonas maltophilia ATCC 53510 in the presence of an amount of nutrient medium sufficient to satisfy nutritional requirements of cells of the culture of Pseudomonas maltophilia ATCC 53510. When the ionic species is other than Hg +   or Hg ++ , other species of Pseudomonas can be used.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Revis et al., copendingapplication Ser. No. 06/918,768, filed Oct. 14, 1986.

TECHNICAL FIELD

This invention relates to the use of Pseudomonas organisms to reduceobjectionable ionic species of heavy metals in waste waters to theelemental form and to thus reduce the concentration of the heavy metalin the waste water.

BACKGROUND ART

Trace elements are widely used in industry and for medicinal purposes.Platinum, mercury, cadmium and lead are used in the plating industry,for example, to make electrodes for batteries and lamps. Compounds ofthese metals are used as catalysts for making varnish and paintcompositions.

Waste waters from mining and public utilities also contain toxic heavymetals or their compounds. In addition, disposal of radionuclides fromaqueous wastes of nuclear power plants is an important problem.

Disposal of wastes from these industries presents an ecological problem,particularly when the heavy metals are in the form of organic compoundsor complexes, such as methylated mercury or lead. Organometalliccompounds in waste waters are generally considered more objectionablethan inorganic metal compounds or elemental metals because consumptionof waters, containing organometallic compounds, by humans or domesticanimals results in absorption and accumulation of relatively largeamounts of heavy metal compounds in the organs of the animals, whichconsume the water. Organometallic compounds are absorbed from theintestinal tract much more readily than inorganic metal compounds or thefree metals themselves. Moreover, organometallic compounds can cross theblood-brain barrier, accumulate in nervous tissue and cause disorders ofthe nervous system. Therefore, there is considerable interest inprocesses for removing toxic or objectionable heavy metal species fromwaste waters.

Summers et al., "Volatilization of Mercuric Chloride byMercury-Resistant Plasmid-Bearing Strains of Escherichia coli,Staphylococcus aureus, and Pseudomonas aeruginosa," J. Bacteriol., vol.113 (1973), pages 1070-1072, have recited that strains PU21/FP,PU21/Stone and PU21/PS18 of Pseudomonas aeruginosa appear to convertmercuric chloride to elemental mercury, or another mercurialcomposition, which is volatile and soluble in organic solvents.

Chakrabarty et al. (U.S. Pat. No. 3,923,597) have proposed usinggenetically-engineered Pseudomonas bacteria to concentrate mercury orits compounds from liquid streams.

Walker et al., "Mercury-Resistant Bacteria and Petroleum Degradation,"Appl. Microbiol., vol. 27 (1974), pages 285-287 and Kondo et al.,"Mercury and Cadmium Resistances Mediated by the Penicillinase Plasmidin Staphlococcus aureus," J. Bacteriol., vol. 117 (1974), pages 1-7,recite characteristics of mercury-resistant organisms.

Bopp, in U.S. Pat. No. 4,468,461, has proposed using Pseudomonasfluorescens NRRL B-12596 to reduce chromate ions to chromic ions inaqueous solution. The chromic ions are precipitated at about neutral pH.

Kauffman et al. (U.S. Pat. No. 4,519,912) and Baldwin et al. (U.S. Pat.No. 4,519,913) have proposed removing objectionable water-solublespecies from aqueous solutions by treatment with various bacterialorganisms. The proposed processes rely upon reduction of the ionicspecies to a corresponding elemental metal.

It is therefore apparent that removal of objectionable and toxic ionicor organometallic species of heavy metals from waste waters is ofcontinuing importance in recycling of waste waters to provide watersupplies, which are safe for drinking or industrial uses.

It is an object of this invention to provide processes by which toxicand objectionable heavy metal species can be removed from waste waters.

DISCLOSURE OF INVENTION

A process for reducing the concentration of ionic species of at leastone heavy metal, other than mercury, in an aqueous waste solution byconversion to one or more corresponding elemental metals, comprisescontacting the aqueous waste solution containing ionic species of atleast one heavy metal with a culture of a Pseudomonas organism in thepresence of an amount of nutrient medium, sufficient to satifynutritional requirements of cells of the culture of Pseudomonas.

This invention further relates to a process for reducing theconcentration of ionic species of mercury in an aqueous waste solutionby conversion to elemental mercury, comprising contacting the aqueouswaste solution with a culture of Pseudomonas maltophilia ATCC 53510,otherwise as above.

In another aspect, this invention relates to a process for reducing theconcentration of ionic species of zinc or cadmium in an aqueous wastesolution by conversion to elemental zinc or cadmium or by accumualtionin or on cells of microorganisms, by contacting the aqueous wastesolution with a culture of Pseudomonas, as above.

In yet another aspect, this invention relates to a biologically pureculture of a strain of Pseudomanas maltophilia, having the identifyingcharacteristics of Pseudomonas maltophilia ATCC 53510. For the practiceof this invention, a culture of the microorganism designated Pseudomonasmaltophilia ATCC 53510 has been deposited with the American Type CultureCollection, Bethesda, Md.

In the event that during pendency of this application, the Commissionerof Patents and Trademarks shall determine that some individual isentitled to receive progeny in this strain in accordance with theprovisions of 37 C.F.R. 1.14 and 35 U.S.C. 122, the required writtenauthorization will be provided by the assignee of this application.

Upon the issuance of this application as a patent, a culture of thisstrain can be obtained from the permanent collection of the AmericanType Culture Collection, Bethesda, Md.

Pseudomanas maltophilia ATCC 53510 and Pseudomonas species, generally,require a minimal growth medium containing arginine, methionine,glucose, betaglycerophosphate, alanine, phenylalanine, serine, valine,sodium chloride, potassium chloride, magnesium sulfate and ammoniumsulfate. The minimal medium also contains a buffer, preferably2-amino-2-hydroxymethyl-1,3-propanediol, also known as TRIS buffer.

The minimal medium preferably contains, per liter:

    ______________________________________                                        grams                                                                         ______________________________________                                        5-10 g           TRIS                                                         0.05-0.2         ammonium sulfate                                             0.05-0.2         arginine                                                     0.05-0.2         methionine                                                   0.5-2            sodium chloride                                              0.5-2            potassium chloride                                           0.1-0.5          magnesium sulfate                                            2.5-10           glucose                                                      0.5-1            beta-glycerophosphate                                        0.1-1            alanine                                                      0.5-1.5          phenylalanine                                                0.15-0.5         serine                                                       0.05-0.5         valine                                                       ______________________________________                                    

and the pH is adjusted to about 7.2 with 4N hydrochloric acid.

The foregoing minimal medium is that "sufficient to satisfy nutritionalrequirements of cells of the culture of Pseudomonas maltophilia ATCC53510," as used in the specification and claims.

Contemplated equivalents of the minimal medium, providing sufficientnutrients to support the nutritional requirements of cells of thePseudomanas culture, include, but are not limited to, (a) mineral salts,including sulfates and phosphates; (b) amino acids, including methionineand (c) a carbon source, e.g. glucose.

The aqueous waste solutions are contacted with Pseudomonas maltophiliaATCC 53510 or an equivalent organism at temperatures ranging from about5° to about 35° C. However, for optimum culture growth, it is preferredto treat or contact the waste waters with Pseudomonas cultures at highertemperatures, more preferably from about 25° C. to about 35° C.

The pH of the mixture of culture and aqueous waste solution duringtreatment is preferably near neutrality, that is, pH from about 6 toabout 8. When confluent cultures are mixed with aqueous wastescontaining trace metal species, normal osmolality should also bemaintained.

The process of this invention is utilized for removal of significantamounts of trace metal compounds from waste waters. It is used forreduction of levels as high as about 50 ppm. However, it is preferred tooperate the process using feeds containing from a few ppb to about 25ppm.

The time for contacting the aqueous waste solutions with Pseudomonasorganisms is selected to accomplish reduction of the objectionable ionicheavy metal species to the desired extent. The time required can be aslittle as 5-10 minutes when the aqueous waste is treated with confluentcells. More prolonged treatment times, of the order of 12 hours or more,may be required when the aqueous waste solutions are being treated withgrowing cultures of Pseudomonas organisms. It will be understood thatone skilled in the art can determine preferred and optimum contact timesby routine experimentation.

"Ionic species of heavy metals," as used in the specification andclaims, includes both organic and inorganic compounds of heavy metals,in which the metal is bonded by an electrovalent bond to another elementor to a carbon-containing radical. Covalently-bonded compounds are alsowithin the scope of this definition. Compounds which can be treated bythe process of this invention include, but are not limited to chlorides,bromides, acetates, propionates, naphthenates, benzoates, nitrates, andphenolates of cations such mercury, lead, cadmium, zinc, silver, gold,chromium, platinum, and copper in various positive-valence states.Treatment of selenium-containing compounds, e.g., sodium or potassiumselenate, is also included. It will be understood that the process ofthis invention can also be used to treat organometallic compounds, moreparticularly, compounds in which the metal-carbon bond is covalent.Exemplary of organometallic compounds which can be treated by theprocess of this invention are methyl mercury, dimethyl mercury, diphenyllead, dipropyl cadmium, tetraethyl lead and triethyl tin.

"Elemental metal" means the metal in its zerovalent condition.

Waste waters are preferably contacted with the selected Pseudomonasculture under aerobic conditions. The process of the present inventionis accordingly preferably carried out under ambient conditions, withoutexclusion of air from the container in which the treatment is beingdone. Most preferably, air is bubbled through the mixture of waste waterand culturing during the treatment. A preferred rate of air flow forthis type of aeration is 1 ml/min.

In the practice of this invention, mercuric or mercurous ions in wastewaters are reduced to metallic mercury by treatment or incubation withPseudomonas maltophilia ATCC 53510. Other metallic species, particularlyCu⁺⁺, Cr⁺⁺⁺, Hg⁺⁺ or Pb⁺⁺, can be incubated with a variety ofPseudomonas species, preferably Pseudomonas syringae or Pseudomonasdenitrificans. Most preferably, the waste waters containing ionicspecies of these metals, are contacted with a culture of Pseudomonasmaltophilia ATCC 53510.

It has further been found, in accordance with the practice of thisinvention, that selected Pseudomonas organisms, particularly Pseudomonasmaltophilia ATCC 53510, can reduce mixtures, containing several of thespecies Pb⁺⁺, Au⁺, Ag⁺, Cr⁺⁺⁺, As⁺⁺, Hg⁺⁺, SeO₃ ⁼, Pt⁺⁺⁺⁺ and Cu⁺⁺essentially completely to corresponding metals in a reasonable time.Therefore, the method of this invention is useful for reducing theconcentrations of mixtures of toxic or obnoxious ionic species in wastewaters to corresponding metals and for removing significant amounts ofthe metals from the waste solutions.

It has also been found that ionic species of zinc and cadmium arepartially reduced to corresponding metals by Pseudomonas organisms andthat significant accumulation of zinc and cadmium in the organisms ofthe culture serves to reduce the concentration of these ionic species inthe waste water. Therefore, despite the way in which treatment ofwaters, contaminated with Zn⁺⁺ or Cd⁺⁺, operates, the concentration ofthese ions in the waste solution being treated is neverthelessdecreased.

The amount of reduction of various metallic ions in waste waters wasdemonstrated by analyzing incubates by sucrose gradient analysis. Metalcontent of the various sucrose gradients permitted accounting for thedisposition of the metallic ions in a sample, following treatment withthe active cultures.

Treatment of the mixture of waste water and culture can be done in anessentially liquid medium. That is, when the waste water or aqueouswaste solution is relatively free of dispersed particulate matter, theculture is added to the waste water in the form of a broth. Followingthe recommended incubation or treatment period, the incubate can befiltered and reduced metal species can be recovered from the residue onthe filter. Elemental metal can be recovered, for example, by digestingthe residue with nitric acid to destroy the cellular structure of theculture and to dissolve the metals entrained with the cells. Theresulting nitrate solution is treated in conventional ways to recovercorresponding elemental metals.

The waste waters can also be contacted with a supported cultures ofPseudomonas organism. The waste waters can be passed through one or morebeds of microorganisms, immobilized on, or adhered to, beads of plastic,alginic acid or the like. Use of a bed of supported organism ispreferred, because water treatment can be done continuously, withperiodic withdrawal of the beds from service to recover reduced metalspecies therefrom. When the process is being done using supportedmicroorganisms, it will be understood that materials required formeeting the nutrient requirements of the cells of the culture are addedto the feed of aqueous waste. The pH of the feed stream will also beadjusted, as necessary.

In batch processes for treating aqueous waste streams, the concentrationof microorganisms will preferably be at least 10⁸ cells/ml of solution,while for continuous processes, the concentration of organimsms will beat least 10⁴ cells/ml of waste solution. More preferably, concentrationsof cells for continous and batch processes will be above 10¹¹ and 10⁷cells/ml, respectively. The preferred maximum concentration of cellsduring treatment is determined by the rheology of the resulting system.However, in general, the cellular content should not exceed 10¹⁸ and10¹⁵ cells/ml for batch and continuous processes, respectively.

The process of this invention can be carried out in several stages. Whenthe aqueous feed contains high concentrations of heavy metal species, itis preferred to treat the feeds by several treatment steps, each ofwhich results in reducing the concentration of heavy metal species inthe feed.

It has been found that addition of an antibiotic, particular penicillin,ampicillin or 6-aminopenicillanic acid, improves the process. Apreferred level of antibiotic addition is 50 micrograms/ml. Thepreferred antibiotic is ampicillin.

It is proposed that reduction of metal species by the Pseudomonasculture is accomplished by a reductase enzyme, secreted by theorganisms. However, it will be understood that the applicants do notwish to be bound by any proposed theoretical explanations and that theprotection being sought is that, set forth in the appended claims.

BEST MODE FOR CARRYING OUT THE INVENTION

In a most preferred embodiment, the process of this invention is carriedout under aerobic conditions and air is bubbled through a culture ofPseudomonas maltophilia ATCC 53510 and aqueous waste solution. For acontinuous process, a concentration of at least 10⁴ cells/ml ispreferred. For a batch process, a minimum concentration of 10⁸ cells/ml.is preferred.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the following examples, temperatures are set forth uncorrected indegrees Celsius. Unless otherwise indicated, all parts and percentagesare by weight.

EXAMPLE 1 Mimimal Medium for Pseudomonas

Minimal medium concentrate is prepared by dissolving ingredients, shownin Table 1, in 1000 mL of distilled water. The pH of the solution isadjusted to 7.2 with 4N HCl. The concentrate is sterilized byautoclaving.

Medium is prepared by diluting 100 mL of concentrate with 900 mL ofdistilled water.

EXAMPLE 2 Reduction of Ionic Heavy Metal Species by Cultures ofPseudomonas

Pseudomonas maltophilia ATCC 53510 is grown in minimal medium of Example1, to which is added trace amounts of representative heavy metalcations. The initial cellular concentration is 10³ cells/ml of solution.The cultures are incubated at 30° C. for 12 h.

At the end of the incubation period, 2-ml aliquots of incubate areremoved, placed on sucrose gradients and centrifuged. Aliquots of thevarious gradients are removed and analyzed for metal content by atomicabsorption.

Each ionic metal species was incubated with a culture of Pseudomonasmaltophilia ATCC 53510.

Results for samples, containing various metals, are shown in Table 2.The figures shown represent the mean ±SD for each series of threeexperiments.

The procedure used permits separation of medium from cells and heavymetal elements. The density of the heavy metal element is higher thanthat of a corresponding ionic species. Centrifugation of sugar gradientsresults in settling of elemental metals to the bottom layer (50%sucrose), and concentration of the cells in the middle layer (20%sucrose) and of ionic species in the top layer (10% sucrose).

                  TABLE 1                                                         ______________________________________                                        Concentrate for Minimal Media for Pseudomonas**                                                grams                                                        ______________________________________                                        TRIS*              60.5                                                       (NH.sub.4).sub.2 SO.sub.4                                                                        1                                                          Arginine           1                                                          Methionine         1                                                          NaCl               10                                                         KCl                10                                                         MgSO.sub.4         2                                                          Glucose            50                                                         beta-Glycerophosphate                                                                            6.8                                                        Alanine            1.8                                                        Phenylalanine      9                                                          Serine             2.20                                                       Valine             1.2                                                        ______________________________________                                         *2-Amino-2-hydroxymethyl-1,3-propanediol                                      **The solutes in the concentrate were dissolved in 1000 ml of distilled       water and pH was adjusted to 7.2 with 4 N HCl. The concentrate was            sterilized by autoclaving. Minimal medium was prepared by diluting 100 ml     of the concentrate with 900 ml of dist illed water.                      

                  TABLE 2                                                         ______________________________________                                        Distribution of Heavy Metal Species Demonstrated by Sucrose                   Gradient Analysis following Reduction with Pseudomonas.sup.(a)                           % Sucrose                                                                    10(Top).sup.(b)                                                                        20(Middle).sup.(c)                                                                       50(Bottom).sup.(d)                                        [as % of total inorganic ions added/                                          ml of culture]                                                      ______________________________________                                        Controls.sup.(e)                                                                           89-100                                                           PbCl.sub.2 2 μg/mL                                                                     5 ± 2   15 ± 1  80 ± 5                                   AgCl.sub.2 20 μg/mL                                                                    1 ± 1     6 ± 0.5                                                                             93 ± 6                                   AuCl.sub.2 20 μg/mL                                                                      2 ± 0.1                                                                              5 ± 2   93 ± 12                                 HgCl.sub.2 20 μg/mL                                                                    4 ± 2   10 ± 1  86 ± 6                                   Na.sub.2 SeO.sub.3 3 μg/mL                                                             8 ± 4   28 ± 4   64 ± 15                                 PtCl.sub.2 20 μg/mL                                                                    5 ± 1   10 ± 3  85 ± 9                                   CuCl.sub.2 20 μg/mL                                                                    3 ± 2    7 ± 6  90 ± 4                                   CdCl.sub.2 20 μg/mL                                                                    25 ± 5  40 ± 1   35 ± 13                                 ZnCl.sub.2 20 μg/mL                                                                    50 ± 8  38 ± 4  12 ± 2                                   ______________________________________                                         .sup.(a) Results represent mean ± SD                                       .sup.(b) Metal content of medium layer                                        .sup.(c) Metal content of bacterial layer                                     .sup.(d) Elemental metal layer                                                .sup.(e) Samples incubated with metals and without Pseudomonas culture   

The culture-free samples (controls) have a high concentration (90-100%)of heavy metal ion in the top layer. Many incubated samples, containingactive Pseudomonas culture, show high concentrations of elemental metal,as indicated by high metal content in the bottom (50% sucrose) gradient.These results show that Pb⁺⁺, Ag⁺, Au⁺, Hg⁺⁺, SeO₃ ⁼, Pt⁺⁺ and Cu⁺⁺ arereduced to the greatest extent by Pseudomonas maltophilia ATCC 53510.Results for the incubation of Cd⁺⁺ and Zn⁺⁺ with Pseudomonas maltophiliaATCC 53510 suggest that concentration of these metals, in some form, inthe cellular material, is significant and that some reduction toelemental metal also occurs.

EXAMPLE 3 Extent of Reduction of Ionic Species as a Function ofIncubation Time

Cultures are incubated at 30° C. as in Example 2. The initialconcentration of Pseudomonas maltophilia ATCC 53510 is 10³ cells/ml.Aliquots are removed after 4, 8, 12 and 24 h and subjected to sucrosegradient analysis. Aliquots from the 50% sucrose layer are removed andanalyzed by atomic absorption spectroscopy for heavy metal content. Analternative analysis of samples is by filtration of an aliquot through a0.45 micron filter, which is digested in nitric acid to determine metalcontent.

Results are given in Table 3. The first set of numbers corresponds tomean ±SD for three sugar gradient experiments. The numbers inparentheses correspond to results from filtration studies, also fromexperiments in triplicate.

These experiments show that increased incubation time is associated withincreasing reduction of ionic metallic species to elemental metal. Theseresults further show that, after 12 hours' incubation, most of thereadily-reducible metallic species have been reduced. As in Example 2,Cd⁺⁺ and Zn⁺⁺ were not as readily reduced. For correspondingculture-free experiments (controls), less than 4% of the ionic speciesare detected in the 50% sucrose gradient.

                  TABLE 3                                                         ______________________________________                                        Atomic Absorption and Membrane Filtration Analyses                            of Extent of Reduction of Heavy Metal Ions                                    by Pseudomonas as a Function of Time.sup.(a)                                           Incubation Time.sup.(h)                                                       4      8        12        24                                                  [% of total inorganic ion added/ml of culture]                       ______________________________________                                        PbCl.sub.2 2 μg/mL                                                                    40 ± 3                                                                              60 ± 4                                                                              75 ± 7                                                                              79 ± 11                                         (36)     (71)     (70)    (83)                                     AgCl.sub.2 20 μg/mL                                                                   60 ± 2                                                                              80 ± 6                                                                              90 ± 8                                                                             83 ± 8                                           (51)     (68)     (88)    (87)                                     AuCl.sub.2 20 μg/mL                                                                   65 ± 4                                                                              89 ± 9                                                                              90 ± 8                                                                             90 ± 9                                           (69)     (74)     (86)    (81)                                     HgCl.sub.2 20 μg/mL                                                                   50 ± 4                                                                              60 ± 5                                                                              80 ± 4                                                                             85 ± 6                                           (55)     (60)     (73)    (73)                                     Na.sub.2 SeO.sub.3 3 μg/mL                                                            35 ± 1                                                                              46 ± 5                                                                               59 ± 10                                                                           48 ± 4                                           (21)     (36)     (42)    (40)                                     PtCl.sub.2 20 μg/mL                                                                   20 ± 6                                                                              60 ± 8                                                                               79 ± 14                                                                           89 ± 8                                           (29)     (71)     (70)    (71)                                     CuCl.sub.2 20 μg/mL                                                                   71 ± 3                                                                               79 ± 10                                                                            88 ± 6                                                                             90 ± 9                                           (55)     (89)     (80)    (83)                                     CdCl.sub.2 20 μg/mL                                                                   10 ± 2                                                                               9 ± 3                                                                              30 ± 9                                                                             40 ± 8                                           (15)     (25)     (49)    (50)                                     ZnCl.sub.2 20 μg/mL                                                                   15 ± 8                                                                              10 ± 4                                                                              13 ± 7                                                                             15 ± 3                                           (38)     (29)     (30)    (24)                                     Controls.sup.(b)                                                                         <4       <4       <4      <4                                       ______________________________________                                         .sup.(a) Results are shown as mean ± SD for atomic absorption analysis     of 50% sucrose layer (heavy metal element); results in parentheses are        from membrane filtration analysis.                                            .sup.(b) No Pseudomonas added.                                           

EXAMPLE 4 Effect of Aeration on Reduction of Ionic Metal Species

Samples containing a mixture of heavy metal ions are incubated withPseudomonas maltophilia ATCC 51510 (10³ cells/ml) at 30° C. for 12 h. Inone series of experiments, air is bubbled through the mixture of cultureand sample at a rate of 1 ml./min. In the other series, no air wasadded, but the cultures were incubated in the ambient atmosphere.

At the end of 12 hrs' incubation, 4-ml aliquots are removed andfiltered. The residue on the filter is digested in nitric acid andanalyzed by atomic absorption for metal content. Results shown in Table4 are for experiments in triplicate. The numbers given represent mean±SD.

Controls (culture-free sample) are analyzed by sucrose gradientanalysis. The controls have less than 6% of the metal, initiallypresent, in the form of elemental metal in the 50% sucrose layer.

These results show that bubbling air through the cultures and sampleimproves reduction of ionic species to corresponding metals and that allof the readily-reducible metals in the mixture, Pb⁺⁺, Ag⁺, Au⁺, Hg⁺⁺,Pt⁺⁺ and Cu⁺⁺, are reduced equally well.

EXAMPLE 5 Determination of Optimum Conditions for Reduction

Maximum reduction of ionic species of heavy metal ions is facilitated byusing minimal medium, containing TRIS buffer, ammonium sulfate,arginine, methionine, sodium and potassium chlorides, magnesium sulfate,glucose, betaglycerophosphate, alanine, phenylalanine, serine andvaline.

For a continuous process, incubation at a level of at 10⁷ cells/ml ispreferred. For a batch process, the culture level is preferably 10¹¹cells/ml.

Bubbling air through the mixture of sample and culture improves theconversion of ionic species to elemental metal.

Addition of an antibiotic, such as ampicillin (50 micrograms/ml) alsobenefits the process. Contemplated equivalent antibiotics are penicillinand 6-aminopenicillanic acid at levels of 100 micrograms/ml.

                  TABLE 4                                                         ______________________________________                                        Effect of Aeration on Reduction of Heavy                                      Metal Ions by Pseudomonas.sup.(a)                                                      With air     Without air                                                      bubbling     bubbling                                                         [% of total inorganic ion added/mL of culture]                       ______________________________________                                        PbCl.sub.2 2 μg/mL                                                                    93 ± 6      70 ± 7                                           AgCl.sub.2 20 μg/mL                                                                   98 ± 2      99 ± 8                                           AuCl.sub.2 20 μg/mL                                                                   100 ± 8     90 ± 5                                           HgCl.sub.2 20 μg/mL                                                                   90 ± 4      80 ± 9                                           PtCl.sub.2 20 μg/mL                                                                   94 ± 6      70 ± 3                                           CuCl.sub.2 20 μg/mL                                                                   100 ± 3     80 ± 7                                           Control.sup.(b)                                                                          <6             <6                                                  ______________________________________                                         .sup.(a) Mean ± SD.                                                        .sup.(b) No Pseudomonas added                                            

EXAMPLE 6 Treatment of Organometallic Contaminants

Cultures of Pseudomonas maltophilia ATCC 51510 are grown in the presenceof methyl mercury, tetraethyl lead or tributyl tin for 48 h at 32° C.without aeration. Two-ml aliquots of the resulting mixture are filteredor placed on a sucrose gradient. The following metals (as % of amountadded) are found on the filter or in the 50% layer of the sucrosegradient, respectively: Hg, 66%, 48%; Pb, 45%, 39% and Sn 38%, 44%. Incontrol experiments, 90-100% of the metals are recovered in the filtrateor in the top layer of the sucrose gradient.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of this invention to adapt it to various usages andconditions.

What is claimed is:
 1. A process for reducing the concentration of ionicspecies of mercury in an aqueous waste solution by conversion toelemental mercury, comprising contacting the waste solution containingionic species of mercury with a culture of Pseudomonas maltophilia ATCC53510 in the presence of an amount of nutrient medium sufficient tosatisfy nutritional requirements of cells of the culture of Pseudomonasmaltophilia ATCC
 53510. 2. The process of claim 1, wherein the aqueouswaste solution is contacted with the culture of Pseudomonas maltophiliaATCC 53510 at a temperature from about 5° to about 35° C.
 3. The processof claim 1, wherein the aqueous waste solution is contacted with theculture of Pseudomonas maltophilia ATCC 53510 at pH from about 6 toabout
 8. 4. The process of claim 1, wherein the ionic species of mercuryis Hg⁺⁺.
 5. The process of claim 1, wherein the aqueous waste solutionis contacted with the culture of Pseudomonas maltophilia ATCC 53510under aerobic conditions.
 6. The process of claim 1, wherein air isbubbled into a mixture of the aqueous waste solution and the culture ofPseudomonas maltophilia ATCC
 53510. 7. The process of claim 1, whereinthe aqueous waste solution is contacted with the culture of Pseudomonasmaltophilia ATCC 53510 in an essentially liquid medium, products fromthe thus-contacted aqueous waste solution are filtered to separate afiltrate from a solid residue and elemental mercury is recovered fromthe thus-separated solid residue.
 8. The process of claim 1, wherein theaqueous waste solution is contacted with a culture of Pseudomonasmaltophilia ATCC 53510 affixed to a support.
 9. The process of claim 1,carried out as a batch process in the presence of at least 10⁸Pseudomonas maltophilia ATCC 53510 cells/ml of solution being treatedand wherein air is bubbled into a mixture of aqueous waste solution andculture.
 10. The process of claim 1, carried out as a continuous processin the presence of at least 10⁴ Pseudomonas maltophilia ATCC 53510cells/ml of solution being treated and wherein air is bubbled into amixture of aqueous waste solution and culture.